help me with my ROI!

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Hi BP. 

So, I'm looking at a possible deal for a duplex. It's looking like I may be able to get it for 80K and I'm putting 20% down ($16,000) with a conventional loan. After closing costs and some minor cosmetic fixes I'll be in $20K. The ARV is $115K. It's currently rented at $850 and $825 for each unit so $1675 monthly. Management fees are a typical 10%/month, mortgage $442 (including taxes and insurance, $100 water/garbage for a total of $709 in monthly operating costs. I'm at $966/month, $11,592/annually. Does that actually make my ROI 57%? I feel like that can't be right.....

What am I missing?

Thanks!